F. Bruning et al., ELECTRON-ATTACHMENT TO XCN (X=BR,CL) - COMPETITION BETWEEN X(-) AND CN- FORMATION, Journal of physical chemistry, 100(51), 1996, pp. 19740-19746
Electron attachment to BrCN and ClCN is studied in a crossed beam expe
riment. Relative cross sections for the formation of negative ions in
the energy range 0-15 eV are reported. The kinetic energy release of f
ragment ions is studied by means of a time-of-flight (TOF) analysis. B
oth target molecules effectively capture low-energy electrons (<0.5 eV
), leading to the complementary dissociative attachment (DA) channels
X(-) + CN (a) and X + CN- (b). From the shape of the ion yield curve,
the temperature behavior, and ab initio calculations, it is concluded
that in both BrCN and ClCN the DA channels a and b originate from prec
ursor ions with a different electronic configuration: channel a correl
ates with a (2) Sigma state leading to direct electronic dissociation
and channel b to a (2) Pi. state associated with vibrational predissoc
iation. The ions X(-) and CN- also appear from further, comparatively
weak resonances at higher energies. The TOF analysis reveals that only
the products Br- + CN appear with appreciable kinetic energy (2.25 eV
) from a resonance between 4.5 and 8.5 eV. For the three other channel
s (Br + CN-, Cl- + CN, Cl + CN-), however, most of the total excess en
ergy (amounting up to 7-8 eV) appears as internal energy of CN or CN-.
In the system BrCN, the Br- intensity strongly increases with the gas
temperature while the CN- intensity strongly decreases as expected fr
om the endothermicity of reaction a and the exothermicity of reaction
b. In ClCN both DA channels are endothermic. While the intensity of Cl
- increases with the gas temperature, the CN- intensity decreases abov
e 600 K. This behavior is explained by the (temperature-dependent) com
petition between autodetachment and the comparatively slow vibrational
predissociation process in the transient molecular anion.